Wave filter



1942- w. P. MASON 2,271,200

WAVE FILTER.

Filed Jul 19, 1959 ATTORNEY /NVENTOR WP MASON Patented Jan. 27, 1942 WAVE FILTER Warren P. Mason, West Orange, N. -J., assignmto Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 19, 1939, Serial No. 285,237

20 Claims.

This invention relates to wave filters and more particularly to those which employ as reactance elements metallic vibrators electromechanically coupled to the circuit by means 'of piezoelectric plates.

An object of the invention is to improve the transmission characteristics of wave filterswhich employ metallic vibrators as reactance elements.

A feature of the invention is a metallic vibrator with electromechanical coupling means including a pair of piezoelectric plates associated with two sides of the vibrator and a second pair of I plates associated with the other two sides.

In accordance with the invention there are provided wave filters of inherently either high or low image impedance which employ one or more electromechanical vibratory elements of special construction. The element consists of a metallic vibrator preferably of rectangular crosssection, a pair ofpiezoelectric plates associated with two sides of the vibrator for producing longitudinal vibrations therein and a second pairof piezoelectric plates associated with the other two sides of the vibrator for picking up the 'mechanical vibrations and reconverting them into electrical vibrations. Each plate, of course. has associated therewith a suitable electrode for making electrical connection therewith. When the plates are thus associated with ifferent sides of the vibrator they may be located centrally with respect to the length of the vibrator and in this way the introduction of extraneous modes of vibration is minimized. The vibrator is preferably made of a metal or an alloy having a low temperature coefiicient of frequency. The plates may be made from any piezoelectric material but plates of Rochelle salt are preferred because of their high piezoelectric activity.

The vibrator is preferably supported at a node of motion. When thefundarnental frequency is utilized such a node will be found at the center of the vibrator and therefore the vibrator is preferably supported at this point. When the piezoelectric plates are also located at the center of the vibrator the support may conveniently contact the vibrator at a corner so as not to interfere with the plates.

In order to increase the band width obtainable with the filter it'is desirable to increase the electromechanical coupling between the plates and the. vibrator. This coupiingmay be increased by removing material from the vibrator along its longitudinal axis. The piezoelectric driving force will remain the same but the effective elastic constant of the vibrator will be decreased, and

therefore the iezoelectric-mechanical,coupling will be increased. The resonant frequency of the vibrator and the static impedance of the crystal of a tube having a square or rectangular crosssection.

The filter circuits disclosed are of the bridged-T type in which the series arms of the T are constituted by an electromechanical vibrator of the type described above. Two oppositely disposed plates of the vibrator are connected together and l to an input terminal of the filter, the other two plates are connected together and to the corresponding output terminal and the metallic vibrator is connected to the remaining filter terminals. A bridging impedance branch, usually including a capacitor, is connected between the two sets of plates. Two shunt end capacitors are connected, respectivelm'between each set ofplates and the metallic vibrator. The circuit is completed by the addition of two end inductors, connected in series for the low impedance filter and in shunt for the high impedance filter. In a modified form of the invention two electromechanical vibrators. connected in parallel, are used in a filter circuit.

The filters are of the wide-band variety; that is, they have transmission bands which extend between selected cut-oil frequencies. When a single electromechanical vibrator is used the filter will have a peak of attenuation at infinite frequency in the case of the series-connected end inductors, and at zero frequency in the case of the shuntconnected end inductors. In addition, in either case, there will be a second peak which may be placed either above or below the transmission band, depending upon the poling of the; piezoelectric plates. If mutual inductance is provided between the end inductors the .peaks at zero or infinity may be moved in toward the transmission band, or even located on the other side of the band. In the filter employing two vibratory elements a peak of attenuation may be provided on each side of the transmission band, and if mutual inductance is used three arbitrarily placed peaks are obtainable.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying draw mg, of which:

Fig. 1 shows one form of the electromechanical vibratory element of the invention;

plates are not changed by this modification. In

the limiting case the vibrator may take the form Fig. 2 is a cross-sectional view taken at the center of the element of Fig. 1; a

Fig. 3 shows the element of Fig. 1 supported at a comer;

Figs. 4 and 5 are cross-sectional views of modifled elements in which material has been removed along the longitudinal axis of the vibrator;

Figs. 6 and 7 show, respectively, low impedance and high impedance filter circuits in accordance with the invention each employing a single elec-- tromechanical vibratory element; and Figs. 8 and 9 show, respectively, low and high impedance filter circuits each employing two else;

' tromechanical vibratoryelementa.

Referring to the drawing in more detail, Fig. -1

is a side view and Fig. 215 a'cross-sectionalfview taken along the,line:2--2- of brief forrn' oi the electromechanical vibratory element- Thefdevice comprises a metallic vibrator II in the iorm of a bar of square cross-section, a pair of piez'o electric plates I2, l3 with associated'electrodes.

ll, 15 secured to two opposite faces otthe vibrator and a second pair oi 'piezoelecta'lc lates" Hi, I] with associated electrods ldji to the other two faces. The vibrator ll preferably made of an alloy having,allow;tempera"- ture coefficient of frequency such, torrexample,

as one consisting of approxim Mikimt,

trodes "the 29- of the vibrator.; The two series inductors L1, L1 connected at the respective ends of the filter complete the circuit,,,.1fhe path between terminals 2 and 4 may fbegrounded or otherwise fixed in potential.

. By properly proportioning the component ele-' ments the filter of Fig.6 may be designed to transmit freely a band extending between two preassigned cut-oil frequencies. There will be a peakoi attenuation at infinity and a second "eith fibdveor below the transmission band. Iffgall-of the';'orystal plates are so poled that when" the vibrator ll; expands a positive charge f. is generated on the surface of eachplate the secondpeak will appear at some frequency be- Iow 'the lower cut-ofi. On the other hand, if

' an expansion of-the' vibrator creates a positive tor. The plates are made of .anysuitablejpiezoe electric materialbut-Rochellesalt referred due to its high piezoelectric activity, 3

For best results the vibrator-4,;

mode is used a. node is to Therefore, as shown in t e -.rwiew. of Fig. 3. the vibrator; l l; is,-preierablysupportedat a corner, to; of a screw 21 which projects 2 and? screws into a tapped. holein the shownat 23. By supporting lthe ratortat a;

vib

corner the central support does the plates, which are Harmonics of the fundamentalgfrequencm-may; of course,beutilized,if-desired. I ,3, 1231.

The cross-sectional view oLmrJs OwsJ modified form of the vibrator H whichisihesame as the one shown in Figs. 1.;and 2 except that material has been removed along the longitudinal axis, as indicated by the hole 25. ;.Building the vibrator in this form increases the electromechanical coupling between the piezoelectric plates and the vibrator and p rmits the procharge on one pair of oppositely disposed plates and a negative charge on the other pair of plates this peak will occur at a frequency above the upper cut-oil. The location of this peak depends principally upon the value of the bridging capacitorC, and-therefore-this capacitor may-be made variable, as indicated by the arrow, to facilitatethe adjustment of the peak. It mutual inductanceeisv provided between the end inductors, as indicatedby the coemcient of coupling K1, the

peak at infinity maybe moved in toward, or to the lower side of, the band, depending upon the 'slgn-oiQthe-mutual-inductance. It is apparent, v rtherefore',' ;that"by a proper setting of the ca-*' pacitor. C andJa proper choice of mutual inductance'hothrpeaks may be located as desired.

hasqaniin'herently low'image impedance. If a highezimagedmpedanceis .desired the circuit of my in; which the end inductors L2, 11 are in shunt,.is to be preferred. The other 2 component elements in Fig. 7 are the same as mosem '6 but their values may, of course,

be diflerenti, Thefilter ofFig. 7 will ordinarily haven peak of attenuation at zero frequency and a second peak either above or below the band.

dspending upon the poling of the piezoelectric plates .of the element 28, as explained above in i ,connectionwith the filter of Fig, 6. By the introduction of mutual inductance between the two end inductors, as indicated by the coefiicient of vision of a wider transmission band in the filter of which the elementiorms a part. In the limit: in case the vibrator may take the dorm of a tube 26 of rectangular or square cross-section as shown in Fig. 5.

Fig. 6 is a schematic circuit or a wave filter in accordance with the inventionhaving a pair of input terminals I, 2 and a pair of output terminals 3', l and employing as a component part an electromechanical vibratory element 28 of the type described above. The filter is of the symmetrical, imbalanced, bridged-T typein which the series arms of the T are finni'she'd by the, element 28. Two oppositely disposed electrodes l4, is are connected together andto an input terminal I, and the other pairot electrodes l8,

I9 are connected to the correspondingout'put terminal 3. A point 29 of the vibrator is connected to the remaining-filter terminals 2 and coupling K2, the peak at zero may be moved up to any desired frequency, either below or above the transmission band.

The filter circuit of Fig. 8 is similar to that of Fig. 6 except that in the'former two electromechanical vibratory elements 31, 32, connected in parallel, are employed instead of one. Be-

sides apeak of attenuation at infinity the filter will have a peak; on each side of the transmission band i! in one element all of the piezoelectric plates are of the same polarity while in the other element one pair of plates differs in polarity from the other pair. The peak at infinity may be moved down to any desired frequency if mu- 4. A bridging impedance branchincludinga ca- I pacitor C is connected between the two sets 01' electrodes. .Two shunt end capacitors C1, C1 are connected, respectively, between each set of elec- 75 tual inductance is provided between the two end inductors. The filter will have an inherently low image impedance;

Thehigh impedance filter employing two electromechaiiical vibratory elements is shown schematically in Fig. 9. The circuit of Fig. 9 diflers from that of Fig. 8 only in that in the former the end inductors are connected in shunt instead of 'in series. The filter may be designed to have an attenuation peak on each side of the transmission band and a third peak at zero irequency. This latter peak may be moved up to any desired frequency bythe introduction of musaidvibraton; 2. A wave filter havingalpair Iof, .input termi -nals and a pair of output terminals and including an electromechanical vibratory system, a pair of shunt connected capacitors, a pair of inductors connected at the ends of the filter and a bridging impedance branch including a third capacitor connected in an electrical path between one input terminal and the corresponding output terminal, said vibratory system com'prising a mechanical vibrator having four sides, four piezoelectric plates secured respectively to said sides, electrical connections from said one input terminal to two of said plates, electrical connections from said corresponding output terminal to the remaining plates and an electrical con-' nection from said vibrator to. the remaining filtor terminals, and said component elements being so proportioned with respect to each other and with respect to two preassigned cut-off frequencies that the filter has a band of free transmission extending between said frequencies and substantial attenuation outside oi. said band.

3. A wave filter in accordance with claim 2 in whichsaid inductors are connected in series at the respective ends of the filter.

' 4. A wave filter in accordance with claim 2 in which said inductors are connected in shunt at the respective ends of the filter.

5. A wave filter in accordance with claim 2 in which said inductors are inductively coupled.

6. A wave filter in accordance with claim 2 which includes a second similar electromechanical vibratory system connected in parallel with said first electromechanical vibratory system.

7. A vibratory system comprising a mechanical vibrator having four sides, four piezoelectric plates of Rochelle salt secured respectively to said sides, means for making electrical connections to said plates and means for supporting said vibrator on a corner.

8. A vibratory system comprising a mechanical vibrator having -i'our sides, four piezoelectric plates secured respectively to said sides and means for. making electrical connections to said plates, said plates being centrally disposed with respect to the length 01 said vibrator.

9. A vibratory system comprising a mechanical vibrator having four sides, four piezoelectric plates of Rochelle salt secured respectively to said sides and means'for making electrical connections to said plate.

A vibratory system comprising a mechanical vibrator having four sides, four piezoelectric plates secured respectively to said sides, means for making electrical connections to said plates and means for supporting said vibrator on a corner at a node oi motion. A

11. A vibratory system comprising a metallic bar, a pair of oppositely disposed piezoelectric plates of Rochelle salt secured to two sides of said bar for producing longitudinal mechanical vibrations therein, a second pair of oppositely disposedpiezoelectric plates oi-Rochelle salt se-' cured totwo other sides of said bar for picking up said mechanical vibrations and converting theminto electrical vibrations and means for supporting said bar at a corner.

12.-A vibratory system comprising a metallic bar,. a pair of oppositely disposed piezoelectric platesis'ecuredto two sides of said bar for producing longitudinal mechanical vibrations therein and a second pair -of oppositely disposed piezoelectric plates secured to two other sides of said -bar for picking up saidmechanical vibrations and converting them .into electrical vibrations,

material having been removed along the longitudinal axis of said bar to decrease the mass of the bar and thereby increase the electromechanical coupling between the bar and the associated piezoelectric plates. 1

13. A vibratory system comprising a metallic bar, a pair of oppositely disposed piezot lectric plates secured to two Sides-of said bar for producing longitudinal mechanical vibrations therein, a second pair of oppositely disposed piezzoelectric plates secured to two other sides of said bar 'for picking up said mechanical vibrations and converting them into electrical vibrations and means for supporting said bar at a corner.

14. A vibratory system comprising a metallic bar, a pair of oppositely disposed piezoelectric plates secured to two sides of said bar for producing longitudinal mechanical'vibrations therein, a second pair of oppositely disposed piezoelectric plates secured to two other sides of said bar for picking-up said mechanical vibrations and converting them into electrical vibrations and tube of rectangular cross-section and four piezoelectric plates, one of said plates being secured to each of the sides of said tube and said plates being centrally disposed with respect to the length of said tube. I

17. A wave filter in accordance with claim 2 in which said two piezoelectric plates connected to said one input terminal are on opposite'sides of said mechanical vibrator.

18. A wave filter in accordance with claim 2 in which said piezoelectric plates are of Rochelle salt.

19. A wave filter in accordance with claim 2 which includes a second similar electromechanical vibratory system connected in parallel with said first electromechanical vibratory system, one of said systems having piezoelectric plates all of the same polarity and the other of said systems having one pair of plates which difier in polarity from the other associated pairof plates.

20. A wave filter in accordance with claim 2 in which said mechanical vibrator is a metallic bar with material removed along its longitudinal axis to decrease the massof the bar and thereby increase the electromechanical coupling between WARREN- P. MASON.

DISCLAIMER 2,271,200.Warren P. Mason, West Orange, N. J. WAVE FILTER. Patent dalaed January 27, 1942. Disclaimer filed April 22,. 1942, by the assignee, Bell Telephone Laboratories, Incorporated. Hereby enters this disclaimer to claims 1, 8, and 9 of said Letters Patent.

[Qflicial Gazette May 19, 1942,]

DESCLAIMER 2,271,200.Warren P. Mason, West Orange, N. J. WAVE FILTER. Patent dated January 27, 1942. Disclaimer filed April 22,. 1942, by the assignee, Bell Telephone Laboratories, Incorporated.

Hereby enters this disclaimer to claims 1, 8, and 9 of said Letters Patent.

[Ofiim'al Gazette May 19, 1942,] 

